At least 2.5% of fuels on sale on forecourts must be biofuels from today, as part of a Government initiative to make motoring greener.
However, the scheme has already come under attack, as some experts have raised concerns about the impact of biofuels on the environment. Critics include the Government's highest-ranking environment scientist, Professor Robert Watson.
The officials argue that the production of biofuels can actually increase greenhouse gas emissions, particularly in areas where forests are cleared to grow the crops needed to make biofuels.
Watson told BBC Radio 4: 'It would be insane if we had a policy to reduce greenhouse gas emissions which uses biofuels that are actually leading to an increase in greenhouse gases.'
The move is part of the Renewable Transport Fuel Obligation (RTFO), which requires 2.5% of all road fuels to come from biofuels from today, rising to 5% by 2010.
Motorists will fill their vehicles as normal, but the change is expected to save 2.5million tonnes of carbon dioxide by 2010.
Transport Minister Jim Fitzpatrick said: 'Making it easier for motorists to use greener fuel is an important step towards reducing carbon emissions from transport. It should help save millions of tonnes of carbon dioxide in the coming years.
'However, we must do all we can to ensure biofuels are produced sustainably. We know people are concerned about the environmental risks associated with expanding biofuel production and we take those concerns very seriously.'
A review into the indirect impacts of biofuels was recently announced by Transport Secretary Ruth Kelly.